The hardness of plastic materials is typically determined by use of a durometer. A durometer is used for measuring hardness of plastic material such as polyethylene, polypropylene, polyurethane, thermoplastic elastomer tubing and natural rubber. The durometer measures the amount of reflected bounce of a special hammer of the material being tested. Generally, the harder the material, the greater will be the rebound. Plastic tubing, for example, is tested using a durometer in what is known as a Shore A test. A Rockwell hardness test is commonly used to test plastic materials for resistance to indentation. In the test, a diamond or steel ball, under pressure, is used to pierce the test specimen. A 10-kilogram weight is first applied and the degree of penetration noted. The so-called major load (60 to 150 kilograms) is next applied and a second reading obtained. The hardness is then calculated as the difference between the readings and expressed with nine different prefix letters to denote the type of penetrator used and the weight applied as the major load.
Other methods for testing hardness rely on pressing an indenter into a flat sample by means of a load applied to the indenter. After removing the indenter and applied load, an impression is left as a result of deformation and an optical measurement of the width of the impression provides data from which hardness is calculated. For example, U.S. Pat. No. 4,699,000 describes a method for testing the hardness of metals by continuously recording both the applied load and the displacement of the indenter for determining metal characteristics such as modulus, yield stress, impact, hardness or strength, creep, and fatigue. As in the case of the conventional durometer, the method of the 4,699,000 patent is not usable for determining toughness or hardness of delicate or semi-solid materials.
The foregoing techniques are simply not useful for measuring the toughness, firmness or hardness for materials which are extremely delicate or fragile and which are in the form of a thin film or thin layer of coating. For such delicate materials, conventional devices and methods would destroy the sample without revealing anything about its hardness or toughness.
Therefore, what is needed is a new device and method for testing the mechanical properties of delicate polymeric, semi-solid, colloid or gel material in the form of a film or a layer of coating with a thickness of as little as 10 microns.